The present invention relates to the field of digital-to-analog converters which may be utilized in tuning systems for radio and television receivers.
Digital-to-analog converters are utilized to convert binary signals to corresponding analog signals. Such converters have wide application. For example, in an electronic tuning system for a radio or television receiver, a digital-to-analog converter may be employed to convert binary signals, which are read out from a memory when a respective channel is selected, to a corresponding tuning voltage for controlling a voltage controlled frequency selective device such as a varactor diode.
A wide variety of digital-to-analog converters are known for various applications. Tuning systems often employ either a binary rate multiplier or a pulse-width modulator type of digital-to-analog converter. Each of these types converts binary signals representing the magnitude of an analog voltage to a pulse signal which is filtered by a low pass filter to derive the analog signal. The pulse signal generated by a binary rate multiplier includes uniformly short duration pulses proportional in number to the magnitude of the analog signal. The pulse signal generated by the pulse-width modulator generally includes a single pulse having a duration proportional to the magnitude of the analog signal.
Since the pulse signals generated by binary rate multipliers include a relatively large number of relatively short duration pulses, they can be readily filtered by low pass filters having relatively high cut-off freqencies and corresponding relatively short response times. However, since the rising and falling transitions of semiconductor devices change in a non-compensating manner as a function of temperature, binary rate multipliers which generate pulse signals with a relatively large number of transitions (i.e., there are two transitions for each pulse) are not stable with respect to temperature. Pulse-width modulators are less susceptible to temperature variations than are binary rate multipliers since the pulse signals they generate include only a few, e.g., two, transitions. However, because of the relatively long duration of the pulses generated by pulse width modulators, the associated low pass filter must have a relatively low cut-off frequency. Accordingly, pulse-width modulators have relatively slow response times. In addition, because low pass filters with relatively low cut-off frequencies may require a relative large-value filter capacitor, pulse-width modulators do not lend themselves to complete incorporation in an integrated circuit.
Since temperature stability and filter considerations are both important in tuning systems, digital-to-analog converters which have the advantages but not the disadvantages of binary rate multipliers and pulse-width modulators are desired. In other words, a digital-to-analog converter for use in tuning systems, as well as other applications, is desired which generates a pulse signal having a compromise number of pulses somewhere between the relatively large number of pulses generated by a corresponding binary rate multiplier for ease of filtering and the relatively low number of pulses generated by a corresponding pulse-width modulator for temperature stability.